1. Field of the Invention
This invention relates to the restoration and maintenance of eutrophied natural bodies of water such as lakes.
2. Description of the Prior Art
There is a natural biological aging of inland lakes known as eutrophication when nutrients are added by rainfall, erosion, or by inflowing waters. The human population has increased this rate of nutrient addition, in many cases, to a level where the natural controls are overloaded and accelerated aging results.
Both plants and animals require a proper supply of nutrients for growth. The amount of organic life that can be supported in a lake or stream is dependent upon the amount of nutrients available.
Major nutrients include nitrogen, oxygen, carbon, silicon, chlorine, calcium, potassium, magnesium, iron, copper, sulphur, phosphorus and boron. In addition, trace quantities of elements such as iodine, cobalt, chromium, copper, zinc, manganese, selenium, vanadium and molybdenum are necessary for most living species and should be present in the water, the sediments, or the food chain.
Eutrophic means "well nourished" and the amount of plant and animal life that a lake can support is dependent upon the amount of nutrients in the internal biological cycles and supplied from the watershed. Unfortunately, an excessive supply supports excessive growth and causes changes to less desirable species, so that a lake is brought to an early death rather than the very slow development which nature intended.
It has been found that growth is limited when any one of the major nutrients is in short supply. The most usual limiting nutrient is phosphorus. Consequently, growth can be controlled by limiting the amount of phosphorus within the biological cycles, and even where phosphorus is not the present limiting nutrient, it can be made so by sharply reducing the amount available.
As a nutrient, phosphorus is in the form of phosphoric acid. Some enters the lake as the acid from the watershed. More is generated within the lake by the action of decay bacteria upon dead plant and animal material, some of which has been delivered from the watershed, for example by streams and drains. In solution in water it has the dissociated form: EQU H.sub.3 PO.sub.4 +H.sub.2 PO.sub.4.sup.-1 +HPO.sub.4.sup.-2 +PO.sub.4.sup.-3
It is in this form that both algae and rooted plants absorb it and thereafter convert it for their own use.
A sign of a eutrophied lake is the appearance of measurable quantities of blue-green algae (cyanophyceae). By definition, a eutrophic lake, when phosphorus is the limiting nutrient, is a lake with more than 0.020 milligrams per liter or phosphorus in the water. Ten micrograms or less per liter is the desirable level for a clean lake.
Although various attempts have been made to restore eutrophied lakes, none has been entirely satisfactory. Such attempts have generally consisted of adding large amounts of a precipitating agent directly to the lake water to precipitate phosphorus. However, these techniques only provide a temporary reduction in the phosphorus levels, since the sediment produced gradually begins to release entrapped phosphorus. In addition, the continuous addition of phosphorus by rainfall, erosion, and inflowing waters continues to raise the phosphorus levels. What is needed is an over-all system for restoring a eutrophied lake and for continually maintaining it in its de-eutrophied state.